1. Field of the Invention
The present invention relates to a disk drive device that drives a recording disk.
2. Description of the Related Art
Recently, disk drive devices such as HDDs have been drastically improved in their performance by being provided with a fluid dynamic bearing. With this improvement, the disk drive devices have been required to be operable at a higher speed. For example, the rotational speed of a disk drive device that magnetically records data has been conventionally 3600/min; however, the speed has been increased to 5400/min and is further being increased to 7200/min.
Herein, the torque ripple and the cogging torque generated by a drive unit that rotationally drives a recording disk may cause the vibration of the disk drive device. For example, Japanese Patent Application Publication No. 2000-312448 discloses an electric motor in which the occurrence of the cogging phenomenon is suppressed.
The torque ripple and the cogging torque include many harmonic components, which are generally increased in proportion to the increased rotational speed. As the harmonic components in the torque ripple and the cogging torque are increased, the vibration of the recording disk is increased.
Large vibration of the recording disk disturbs the tracing of the recording tracks by the magnetic head, causing a failure in reading/writing data, that is, deteriorating the so-called error rate. Accordingly, the rotational speed of the recording disk cannot be increased, and thereby the disk drive device is hampered in being operable at a higher speed.